Intermodulation distortion in passive microwave components is caused by internal nonlinearities. In a truly linear system, the output is directly proportional to the input. In a nonlinear system, the output signal is distorted by changes in the amplitude of the input signal. Intermodulation distortion creates new output signals from the nonlinear combinations of two or more input signals mixed together. A nonlinear circuit will create an infinite number of harmonics from two fundamental frequencies (f1 and f2). A particular concern for telecommunication systems engineers is the Intermodulation product of the third-order (such as 2f1 - f2 and 2f2 - f1), especially if f1 and f2 are closely spaced. With certain system designs and bandwidth allocations, the third order Intermodulation products can be generated at the same frequencies as the receive channels of the system. In general, Intermodulation products increase system noise and reduce the number of available channels.

Intermodulation distortion is most pronounced in systems where the high power transmission and low power receiver signals are carried simultaneously in the same transmission line, such as in the cable between the duplexer and the antenna in GSM base stations and in certain space applications. For low power levels, the effects of Intermodulation distortion are significantly less. Astrolab Inc. is involved in the research of the Intermodulation problem as a participant in the IEC TC46 WG6 Passive Intermodulation Working Group.

Coaxial cable assemblies have often been viewed as linear components. However, pure linear components do not exist. There are small nonlinearities in the connectors and in the cable to connector junctions. Intermodulation distortion in connectors is usually caused by thin-surface oxide layers at the connector junctions or by insufficient contact pressure when the current-carrying contact zones become separated. Separation is usually microscopic and can be caused by either electron tunneling or microscopic arcing. The presence of ferromagnetic materials in the current path may also contribute to intermodulation distortion.

Some simple design rules can help avoid Intermodulation distortions in coaxial cable assemblies:

• Use of semi-rigid cable with a seamless outer conductor in place of flexible cable.
• Use of a solid center conductor in place of a stranded center conductor.
• Directly attach the outer conductor to the connector body by soldering or clamping in lieu of crimping.
• Limit the number of parts in the current path.
• Eliminate contaminants in the current path.
• Use high quality machining in the connector parts with a smooth surface finish.
• Avoid contaminants in the plating solutions.
• Ensure adequate and uniform plating thickness.
• Avoid use of magnetic materials in the current-carrying-path.
• Ensure adequate contact pressure.
• Contact surface of female contact fingers should cover as close to 360° as possible (i.e. narrow slots or slotless).
• Use connector interfaces with radial dimensions as large as possible (7/16 over N, N over SMA).

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